4.4 Point Positioning Errors
Point positioning is impacted by the errors discussed in Module 3. The main error that point positioning resolves is the receiver clock error.
The errors that impact point positioning are:
- Satellite clock errors – these are primarily dealt with by the control segment through the application of corrections.
- Satellite orbit area – while point positioning can resolve a level of orbit errors, it relies on the accuracy of the ephemeris, which is managed by the control segment.
- DOP – GNSS receivers tolerance levels can be set to only accept data when DOP values are below certain levels.
- Low elevations – similarly to DOP, GNSS receivers may have an elevation mask setting that can be adjusted depending on local obstructions.
- Atmospheric delays – point positioning attempts to resolve some atmospheric delays through receiver models, however, single frequency observations like point positioning make resolving these errors difficult. Dual or multiple frequency methods of observation are significantly better at resolving atmospheric errors.
- Obstructions and multipath – these are site dependent and impact point positioning. Users should consider choosing a different location if possible.
- Receiver noise – tolerance levels for SNR can be set in most receivers. Avoidance of electrical influence, such as power lines or similar can assist in reducing receiver noise issues.
- Spoofing – point positioning is highly susceptible to spoofing as the majority of attacks are in the L1 band.
- Human error – our capacity to introduce error into point positioning is infinite, however, it can be mitigated by understanding of how GNSS operates and through development of quality systems to manage observations of positions.
Minimising errors in point positioning
While reducing the errors in point positioning is virtually impossible through observation manipulation, there are observation techniques that allow errors to be minimised or averaged out. These techniques also assist in addressing human error.
Point averaging
GNSS errors can be significantly reduced by averaging individual point positions over time. The amount of time a point should be observed is dependent on the accuracy looking to be achieved, however, in point positioning 10-30 seconds is usual sufficient for 10m accuracy in open areas.
Redundant observations
Revisiting a point multiple times generates redundant observations. These are most easily thought of as checks on the observations taken to assure the user of the data that it is of the appropriate accuracy. Redundant observations should be done at the end of each observation session, before and after rest periods on projects (including overnight) or even periodically throughout a project. Observations should be separated by an appropriate amount of time to ensure systematic errors are reduced.
Measuring known marks
Periodically or throughout a project, a GNSS receiver being used for point positioning should be checked against a mark of known coordinates. This allows the user to compare the receiver coordinates to the known coordinates, checking for any ongoing bias or issues with positioning.